Make OpenSSL::Test a bit more flexible

So far, apps and test programs, were a bit rigidely accessible as
executables or perl scripts.  But what about scripts in some other
language?  Or what about running entirely external programs?  The
answer is certainly not to add new functions to access scripts for
each language or wrapping all the external program calls in our magic!

Instead, this adds a new functions, cmd(), which is useful to access
executables and scripts in a more generalised manner.  app(), test(),
fuzz(), perlapp() and perltest() are rewritten in terms of cmd(), and
serve as examples how to do something similar for other scripting
languages, or constrain the programs to certain directories.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1686)